Perl I/O

Opening files

open CONFIG, "dino.config";       # open for reading
open CONFIG, "<", "dino.config";  # same thing
open BEDROCK, ">", "fred.txt";    # overwrite
open LOG, ">>", "logfile.log";    # append

Read a file line-by-line

open(FH, $fileName) or die "open $fileName failed: $!";
while (my $line = <FH>){
    chomp $line;   # get rid of end-of-line
    # do something with $line
}

A simpler alternative:

open(FH, $fileName) or die "open $fileName failed: $!";
while (<FH>){
    chomp;  # acts on the default variable $_
    # do something with $_
}

Read a file all at once

open(FH, $fileName) or die "open $fileName failed: $!";
chomp(@lines = <FH>);
# process @lines

Write to a file that's been opened for writing

print LOG "Captain's log, stardate 3.14159\n";  # output goes to LOG
printf STDERR "%d percent complete.\n", $done/$total * 100;

Perl's built-in filehandles

STDIN, STDOUT, STDERR allow your Perl program to act like a standard Unix command.

Use select to choose an alternate default output filehandle:

select LOG;
$| = 1;  # don't keep LOG entries sitting in the buffer
select STDOUT;
...
print LOG "This gets written to the LOG at once!\n";

Closing files

close CONFIG;  # Perl automatically closes open files at program exit

Remapping STDERR to a log file

open STDERR, ">>", "/var/log/system.log" or die "$!";

Example: updating several files

#!/usr/bin/perl -w

chomp(my $date = `date`);
$^I = ".bak";

while (<>) {
  s/^Author:.*/Author: Randal L. Schwartz/;
  s/^Phone:.*\n//;
  s/^Date:.*/Date: $date/;
  print;
}

Setting $^I causes the <> to print the updated lines to the original file name and save the original contents as a backup file.